﻿INTKODTJCTKXN". 
  XV 
  

  

  until 
  they 
  pass 
  almost 
  imperceptibly 
  into 
  the 
  typical 
  physostomous 
  

   bony 
  fishes 
  ; 
  and 
  these 
  forms 
  may 
  be 
  arranged 
  in 
  at 
  least 
  three 
  

   families. 
  

  

  This 
  classification, 
  adopted 
  in 
  the 
  Catalogue, 
  is 
  a 
  little 
  different 
  

   from 
  any 
  hitherto 
  proposed, 
  because 
  no 
  great 
  systematic 
  value 
  is 
  

   ascribed 
  to 
  the 
  characters 
  of 
  the 
  scales. 
  When 
  one 
  genus 
  of 
  Meso- 
  

   zoic 
  ganoid 
  (Aetheolepis) 
  is 
  proved 
  to 
  exhibit 
  every 
  variety, 
  from 
  the 
  

   thick, 
  rhombic, 
  firmly 
  articulated 
  scales 
  of 
  the 
  abdominal 
  region 
  

   to 
  the 
  delicate, 
  cycloidal, 
  deeply-overlapping 
  scales 
  of 
  the 
  caudal 
  

   pedicle 
  — 
  when, 
  moreover, 
  thick 
  and 
  thin 
  scales 
  cover 
  respectively 
  

   the 
  abdominal 
  and 
  caudal 
  regions 
  of 
  Tetragonolepis, 
  it 
  is 
  no 
  longer 
  

   possible 
  to 
  depend 
  entirely 
  upon 
  scale-characters 
  for 
  the 
  definition 
  

   of 
  families 
  and 
  orders. 
  Since, 
  therefore, 
  Eugnaihus 
  and 
  Caturus 
  are 
  

   identical 
  in 
  their 
  osteology, 
  the 
  latter 
  merely 
  differing 
  from 
  the 
  former 
  

   in 
  the 
  subdivision 
  of 
  its 
  supraorbital 
  plates 
  and 
  the 
  tenuity 
  of 
  its 
  

   overlapping 
  scales, 
  these 
  two 
  genera 
  are 
  now 
  placed 
  in 
  one 
  and 
  the 
  

   same 
  family. 
  They 
  and 
  their 
  allies 
  constitute 
  the 
  Eugnathidse, 
  

   which 
  range 
  from 
  the 
  Upper 
  Trias 
  to 
  the 
  Upper 
  Cretaceous 
  ; 
  and 
  it 
  

   is 
  difficult 
  to 
  distinguish 
  the 
  more 
  specialized 
  forms 
  from 
  the 
  

   Amiidse. 
  

  

  Both 
  Eugnaihus 
  and 
  Caturus 
  range 
  from 
  the 
  Upper 
  Trias 
  to 
  the 
  

   Kimmeridgian, 
  and 
  the 
  latter 
  probably 
  even 
  to 
  the 
  Wealden. 
  The 
  

   deeper-bodied 
  Heterolepidotus, 
  which 
  is 
  scarcely 
  distinguishable 
  

   from 
  Eugnaihus, 
  also 
  seems 
  to 
  exhibit 
  as 
  wide 
  a 
  range 
  as 
  the 
  latter. 
  

   More 
  specialized 
  genera, 
  however, 
  like 
  the 
  highly 
  ornate 
  Ptycho- 
  

   lepis 
  (Upper 
  Trias 
  — 
  Upper 
  Lias), 
  the 
  delicate, 
  overgrown 
  Osteo- 
  

   rachis 
  (Lower 
  Lias), 
  and 
  the 
  cycloidal-scaled 
  Eurycormus 
  (Oxf 
  ordian 
  

   and 
  Kimmeridgian), 
  are 
  very 
  restricted 
  in 
  their 
  range 
  ; 
  so 
  also 
  are 
  

   the 
  Cretaceous 
  genera 
  Neorhombolepis, 
  Otomiila, 
  and 
  Lophiostomus. 
  

  

  Neorhombolepis 
  and 
  Otomitla 
  are 
  particularly 
  interesting 
  because, 
  

   so 
  far 
  as 
  known, 
  they 
  retain 
  the 
  external 
  characters 
  of 
  Eugnaihus, 
  

   and 
  at 
  the 
  same 
  time 
  possess 
  vertebral 
  centra 
  as 
  completely 
  developed 
  

   as 
  those 
  of 
  Amia. 
  If 
  correctly 
  placed 
  in 
  the 
  Eugnathidaa, 
  they 
  form 
  

   another 
  illustration 
  of 
  the 
  principle 
  noted 
  among 
  the 
  Semionotidas 
  

   and 
  Macrosemiidge, 
  that 
  the 
  most 
  advanced 
  stage 
  of 
  the 
  endo- 
  

   skeleton 
  is 
  attained 
  in 
  the 
  latest 
  members 
  of 
  the 
  race 
  with 
  the 
  

   least 
  modified 
  exoskeleton. 
  Lophiosiomus, 
  which 
  is 
  characterized 
  

   by 
  the 
  fusion 
  of 
  most 
  of 
  its 
  cranial 
  roof-bones, 
  and 
  is 
  provided 
  with 
  

   a 
  pair 
  of 
  bosses 
  on 
  the 
  hinder 
  portion 
  of 
  this 
  roof, 
  seems 
  to 
  exhibit 
  

   only 
  ring-vertebrae. 
  

  

  